Explainer | How do vaccines and immune systems work, and why could a coronavirus vaccine fail?
- Six vaccine candidates have reached phase 3 human trials, but it is not uncommon for vaccines to fall at this final hurdle
- The success of vaccines also comes down to what we expect from them and how long immunity lasts
What is the immune system?
It is a complex system of cells, organs and processes that detect foreign substances, including bacteria, viruses, parasites and transplanted organs. The human body counts on the system to detect and identify the foreign substances then develop layers of defence against them.
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How does the immune system work?
When germs such as bacteria or viruses invade the body, they multiply, causing infection and possible illness. The body can mount a non-specific attempt – called an innate response – to try to eliminate the invaders within hours of an infection. Infections that can be quickly resolved by innate response may not produce symptoms.
When an innate response fails to contain an infection, adaptive response is mounted, usually days later, to fight off the invasion with the help of white blood cells: T-cells and B-cells.
Some types of T-cells identify and kill infected cells. Some help B-cells make antibodies, a kind of protein. A specific type of antibody, called a neutralising antibody, binds to a pathogen and blocks it from entering human cells, thus stopping infection, in a process known as neutralising.
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It is all right for the number of antibodies to decline after a disease is cured, thanks also to T-cells and B-cells. The so-called memory B-cells and T-cells stored in the body can remember the infection and stand by to fight off a similar invasion in future.
How does a vaccine work?
Vaccines train the immune system to develop a response by mimicking an infection. The process may cause symptoms such as fever, but it also prompts the human body to produce T-cells and antibodies. When the mock infection is over, the body is left with memory T-cells and B-cells that hopefully can remember the infection and mount a defence immediately in the face of a real threat.
How do the Covid-19 vaccines work?
Scientists around the world are trying to develop vaccines against Sars-CoV-2 – the official name of the coronavirus that causes the disease Covid-19 – using differing technologies.
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Are Covid-19 vaccine candidates working?
Candidates for which data has been published have been shown to induce neutralising antibodies against the spike protein. But whether they offer real protection can be discovered only after large-scale phase 3 trials, which test the broader safety and efficacy of a vaccine.